1. Field of the Invention
The present invention relates to methods and apparatus for wellbore completions. More particularly, the invention relates to completing a wellbore by expanding tubulars therein. More particularly still, the invention relates to completing a wellbore by separating an upper portion of a tubular from a lower portion of the tubular.
2. Description of the Related Art
Hydrocarbon and other wells are completed by forming a borehole in the earth and then lining the borehole with steel pipe or casing to form a wellbore. After a section of wellbore is formed by drilling, a section of casing is lowered into the wellbore and temporarily hung therein from the surface of the well. Using apparatus known in the art, the casing is cemented into the wellbore by circulating cement into the annular area defined between the outer wall of the casing and the borehole. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, a first string of casing is set in the wellbore when the well is drilled to a first designated depth. The first string of casing is hung from the surface, and then cement is circulated into the annulus behind the casing. The well is then drilled to a second designated depth, and a second string of casing, or liner, is run into the well. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second liner string is then fixed or “hung off” of the existing casing by the use of slips which utilize slip members and cones to wedgingly fix the new string of liner in the wellbore. The second casing string is then cemented. This process is typically repeated with additional casing strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever decreasing diameter.
Apparatus and methods are emerging that permit tubulars to be expanded in situ. The apparatus typically includes expander tools which are fluid powered and are run into a wellbore on a working string. The hydraulic expander tools include radially expandable members which, through fluid pressure, are urged outward radially from the body of the expander tool and into contact with a tubular therearound. As sufficient pressure is generated on a piston surface behind these expansion members, the tubular being acted upon by the expansion tool is expanded past its point of plastic deformation. In this manner, the inner and outer diameter of the tubular is increased in the wellbore. By rotating the expander tool in the wellbore and/or moving the expander tool axially in the wellbore with the expansion member actuated, a tubular can be expanded along a predetermined length in a wellbore.
There are advantages to expanding a tubular within a wellbore. For example, expanding a first tubular into contact with a second tubular therearound eliminates the need for a conventional slip assembly. With the elimination of the slip assembly, the annular space required to house the slip assembly between the two tubulars can be reduced.
In one example of utilizing an expansion tool and expansion technology, a liner can be hung off of an existing string of casing without the use of a conventional slip assembly. A new section of liner is run into the wellbore using a run-in string. As the assembly reaches that depth in the wellbore where the liner is to be hung, the new liner is cemented in place. Before the cement sets, an expander tool is actuated and the liner is expanded into contact with the existing casing therearound. By rotating the expander tool in place, the new lower string of casing can be fixed onto the previous upper string of casing, and the annular area between the two tubulars is sealed.
There are problems associated with the installation of a second string of casing in a wellbore using an expander tool. Because the weight of the casing must be borne by the run-in string during cementing and expansion, there is necessarily a portion of surplus casing remaining above the expanded portion. In order to properly complete the well, that section of surplus unexpanded casing must be removed in order to provide a clear path through the wellbore in the area of transition between the first and second casing strings.
Known methods for severing a string of casing in a wellbore present various drawbacks. For example, a severing tool may be run into the wellbore that includes cutters which extend into contact with the tubular to be severed. The cutters typically pivot away from a body of the cutter. Thereafter, through rotation the cutters eventually sever the tubular. This approach requires a separate trip into the wellbore, and the severing tool can become binded and otherwise malfunction. The severing tool can also interfere with the upper string of casing. Another approach to severing a tubular in a wellbore includes either explosives or chemicals. These approaches likewise require a separate trip into the wellbore, and involve the expense and inconvenience of transporting and using additional chemicals during well completion. These methods also create a risk of interfering with the upper string of casing. Another possible approach is to use a separate fluid powered tool, like an expansion tool wherein one of the expansion members is equipped with some type of rotary cutter. This approach, however, requires yet another specialized tool and manipulation of the run-in string in the wellbore in order to place the cutting tool adjacent that part of the tubular to be severed. The approach presents the technical problem of operating two expansion tools selectively with a single tubular string.
Similar problems with current methods and apparatus for severing a tubular in a wellbore exist regardless of whether the tubular is casing, where the tubular is hung from the casing of a cased wellbore or from the wellbore wall of an open hole wellbore. The tubular or portions of the tubular must often be removed when the tubular becomes corroded or when the tubular is no longer needed within the wellbore (e.g., because a different type of tubular is needed in the wellbore to perform a different function than previously performed). As mentioned above, the current method of running in a severing tool to sever the tubular requires a separate trip into the wellbore, and the severing tool can malfunction. Explosives or chemicals also require a separate trip into the wellbore and are expensive to transport and use, as stated above. Additionally, the casing of the cased wellbore may be damaged by the running in or the functioning of the severing tool, explosives, or chemicals used to sever the tubular.
Temporary plugs are often used within the wellbore to isolate one portion of the wellbore from the remaining portion of the wellbore. Typically, the plug must be set within the wellbore initially, and then the wellbore operation is performed within one of the portions of the wellbore. When it is desired to remove the plug and thus allow unobstructed access to both portions of the wellbore, the plug must be severed and retrieved from the wellbore. Releasing and/or retrieving the plug is often difficult because of debris falling onto the plug during the preceding wellbore operation. There is a need for a temporary plug which does not require retrieval from the wellbore upon completion of the plug's function within the wellbore. There is a further need for a plug which is capable of being released and/or opened in spite of the presence of debris.
There is a need, therefore, for an improved apparatus and method for severing an upper portion of a tubular after the tubular has been set in a wellbore by expansion means. There is a further need for an improved method and apparatus for severing a tubular in a wellbore. There is yet a further need for a method and apparatus to quickly and simply sever a tubular in a wellbore without a separate trip into the wellbore and without endangering the integrity of the casing within the wellbore.